Currently, two major processes, so-called peroxide-curing and water-curing, are being employed in the application of protective coatings such as insulation and jacketing about wires and cables. The peroxide-curing process involves extruding compositions containing an organic peroxide about wires and cables and subjecting the resultant articles to elevated temperatures in order to cure the compositions to crosslinked products. The overall operation requires careful control of the process parameters in order to avoid undue heat and pressure build-up in the extruder. Undue heat and pressure build-up results in premature decomposition of the peroxides which in turn results in crosslinking of the compositions in the extruder. Crosslinking of the compositions in the extruder, commonly referred to as "scorch" necessitates, in extreme cases, stopping the operation and cleaning the extruder. In situations wherein "scorch" occurs but is not as severe, it has been found that the work-life of the ultimate coatings is relatively short. In addition to the processing difficulties of peroxide-curing, the peroxide containing compositions do not have that degree of resistivity to deformation, at normal peroxide loadings, demanded by many ultimate users of insulated and jacketed wire and cable articles.
The water-curing process, on the other hand, involves compositions containing hydrolyzable, silane modified thermoplastic polymers and is more commercially attractive in that a wider latitude in processing conditions is possible. That is, compositions containing water-curable, silane modified polymers can be extruded at temperatures far in excess of maximum processing temperatures used in extruding peroxide containing compositions. Being capable of extrusion at higher temperatures, such compositions can be extruded at faster rates and under lower pressure and consequently are more cost effective.
A disadvantage with respect to the so-called water-curing process is the water sensitivity of the hydrolyzable thermoplastic polymers. During normal handling and processing, these polymers tend to hydrolyze, determined by infra-red spectroscopic analysis; and in the presence of metal carboxylate, silanol condensation catalysts tend to prematurely crosslink. As an illustration, water-curable compositions are generally formulated by admixing, in an extruder, a water-curable, silane modified thermoplastic polymer with a metal carboxylate silanol condensation catalyst such as dibutyltin dilaurate, extruding the resultant composition about a wire or cable and thereafter passing the coated wire or cable through a water bath in order to cure the coating to a crosslinked product. It has been found, however, that upon admixing the water-curable, silane modified thermoplastic polymer with a metal carboxylate silanol condensation catalyst, the resultant composition tends to prematurely crosslink in the extruder. This premature crosslinking, commonly referred to as "scorch" manifests itself in the extruded coating in the form of surface discontinuity and roughness. In some instances, excessive scorching, as previously stated, may cause enough of a pressure build-up in the extruder to require a cessation of the entire extrusion operation. Consequently, as a result of the "scorch" problem, the water-curing process has not found wide commercial acceptance in the United States.